You can use a hide-when formula on the paragraph containing the Subscribe button, however you need to understand that in Notes a paragraph includes everything on the same line unless you use a table, in which case the paragraph is limited to the table cell. So you could put your buttons in four cells of a table, and apply this hide-when formula to the subscribe button:
From="my newsletter address";
Note that you will have to exactly match the address as it appears in the From header field.
Hide-when formulas are entered by positioning the cursor anywhere in the paragraph that you want hidden, and bringing up the Text properties box. If you're in Notes 8.x, you can do that by selecting Properties from the Text menu. (I don't recall the menu path on older versions, and I usually just use Ctrl-J as a keyboard shortcut.) Then select the second to last tab in the properties box, click on the checkbox labeled 'Hide paragraph if formula is true', and enter the formula in the box below that.